1. Technical Field
This disclosure relates to a sputtering target and a manufacturing method thereof, the target being used for forming a film of an oxide layer which is a constitution layer of a recordable optical recording medium. Further, this disclosure relates to a recordable optical recording medium manufactured by using the sputtering target, in particular, a recordable optical recording medium which enables a high density recording even in a blue laser wavelength area.
2. Description of the Related Art
In order to provide a recordable optical recording medium which enables recording and reproduction using a laser beam with a wavelength the same or less than that of a blue laser beam, extensive development is underway for blue laser beams which enable ultra-high density recording, along with development of recordable optical recording media to which such blue laser beams can be used.
The present inventors propose the validity of a recording layer containing as a main component a metal or a half metal oxide, in particular bismuth oxide (see Japanese Patent Application Laid-Open (JP-A) Nos. 2003-48375, 2005-161831, and 2005-108396) as a recordable optical recording medium which enables high density recording at a wavelength the same as or less than the wavelength of the blue laser.
The applicant of the present invention discloses in the prior application (Japanese Patent Application Laid-Open (JP-A) No. 2006-247897) a recordable optical recording medium which has a recording layer containing as a main component of the constituent element bismuth and containing bismuth oxide, the recording layer containing one or more elements X selected from B, P, Ga, As, Se, Tc, Pd, Ag, Sb, Te, W, Re, Os, Ir, Pt, Au, Hg, Tl, Po, At, and Cd. The applicant also discloses that the recordable optical recording medium using a film including Bi, B, and oxide has excellent characteristics, and that a sputtering method can be also used as a method for forming this layer.
The sputtering method has been widely known as one of the vapor-phase deposition techniques for thin film deposition. The method is also used in an industrial thin film deposition. In the sputtering method, a target material which has the same component as a film to be deposited is prepared. Normally, argon gas ion which is generated by means of glow discharge in the target material is then allowed to collide with this target material with the result that the constituent atoms of the target material are hammered out, and atoms are accumulated on the substrate with the result that a film is formed. Since oxides generally have high melting points in particular, a method such as vapor deposition is not favorable and thus a radio frequency sputtering that involves application of a high frequency is often used.
Sputtering has produced many results in the manufacturing process and is favorable in a point of through-put. However, in the case where a film made of material containing two or more elements is to be deposited, the resultant film often has a different composition than its sputtering target, thus requiring a consideration for determining the composition of the target. Furthermore, the structure and the quality of the film often differ depending on the form in which the compound constituting the target is contained; thus, it is also required to consider this point.
Furthermore, it is also required to further improve the film deposition rate from the viewpoint of the production cost. For increased film deposition rate, it is required to introduce a large power. In this case as well, it is required to improve the target strength so as to avoid destruction of the target.
As a known technology, for example, Japanese Patent Application Laid-Open (JP-A) No. 11-92922 discloses a Bi-based oxide target as a sputtering target for the formation of a dielectric film. However, this document does not describe a target containing B. If the kinds of constituent elements are different, so is the relationship between the composition and constituent compounds of the aforementioned target and the structure and composition of the deposited film. Consequently, it is required to change the composition of the target. The information disclosed in this document does not constitute a reference of the present invention.
Furthermore, the Publication of the Japanese Patent Application Laid-Open (JP-A) No. 2005-264206 discloses a description of a Bi2O3-based glass-like target containing B2O3. However, the invention in this document inevitably contains SiO2 and is associated with the glass-like target with the result that the invention in the document is different from the present invention.